MicroRNAs (miRNAs) are an abundant class of small non-coding RNAs that regulate diverse biological processes. Recent studies suggest that a cluster of miRNAs, the mir-17-92 polycistron located at 13q31, which contains seven miRNAs including miR-17-5p, miR-17-3p, miR-18a, miR-19a, miR-20a, miR-19b-1, and miR- 92-1, may function as an oncogene. These miRNAs are highly expressed in many malignant lymphomas and other neoplasms that contain amplified 13q31-32 chromosomal fragments. It has been demonstrated that enforced expression of the mir-17-92 cluster cooperated with c-myc expression to accelerate tumor development in a mouse model of human B-cell lymphoma, and that introduction of the expression construct of the mir-17-92 cluster could enhance lung cancer cell growth. However, it is unclear whether enforced expression of this miRNA cluster can induce cancer alone or must cooperate with other gene abnormalities. Moreover, the functional mechanisms through which the miRNAs contribute to the development of cancer are poorly understood. Recently, we observed that the miRNAs from this cluster are frequently overexpressed in various acute leukemias, particularly in MLL-rearrangement leukemia, although the degree of up-regulation varies between individual miRNAs. The overexpression of these miRNAs is usually correlated with the amplification of the miRNA cluster at its genomic locus. Thus, we hypothesize that the miRNAs from the mir- 17-92 cluster play a role in hematopoiesis, and that overexpression of the miRNAs may contribute to the development of leukemia alone and/or by cooperating with other abnormalities, e.g., MLL translocations. We also hypothesize that some of the miRNAs in the cluster may play a greater role than others in leukemogenesis. In addition, we hypothesize that overexpression of the miRNAs contributes to leukemogenesis by negatively regulating expression of their target genes, particularly those that are critical tumor suppressors. We propose to explore the role of each of the miRNAs and of the different combinations of these miRNAs in proliferation and differentiation of hematopoietic progenitor cells, and in leukemogenesis by using two different mouse transplantation models. Finally, we have identified 21 highly likely target genes of the miRNAs from this cluster through our recent preliminary studies, and we will validate further the miRNA- target relationship and study the roles of the validated targets in the development of leukemia by using both in vitro and in vivo methods.